docs: def _compute_forearc_backarc_term(self, C, sites, dists):
          """
          Computes the forearc/backarc scaling term given by equation (4).
          """
          f_faba = np.zeros_like(dists.rhypo)
          # Term only applies to backarc sites (F_FABA = 0. for forearc)
          max_dist = dists.rhypo[sites.backarc]
          max_dist[max_dist < 85.0] = 85.0
          f_faba[sites.backarc] = C['theta7'] +\
              (C['theta8'] * np.log(max_dist / 40.0))
          return f_faba